1. The Hexagonal Close Packed lattice is non-Bravais and like the other common non-Bravais lattice, that of diamond, is common to many interesting elements and compounds.
  2. HCP structures include one allotrope of solid Helium, and the metal magnesium.
  3. Numerical studies on HCP are complicated by the two sublattice nature, and it requires 4 Miller indices.
  4. The rich structure of HCP solid Helium makes the additional effort worthwhile.

Image from an AViz website of Bravais and non-Bravais lattices by Shahar Rozen.